This application is substantially similar to a simultaneously filed application of identical title and inventive entity U.S. Pat. No. 245,426 now abandoned.
This invention relates to a deployment apparatus for the performance of a task on a specimen, especially ultrasonic inspection of metal structures and more especially ultrasonic inspection of piping and pipe welds located in high temperature and high radiation environments.
Since defects in metal and welds can lead to overall failure, routine inspections are required in many applications. Non-destructive inspection using ultrasonic waves is now well known art. The general principle teaches that an ultrasonic wave, introduced into a specimen, is reflected on encountering a change in acoustical impedance. Pores, cracks, and other defect types in metal strongly reflect sound waves, creating echoes.
Ultrasonic testing equipment introduces sound waves of frequency above 100,000 cycles per second into the specimen to be inspected and records on an oscilliscope style screen the pulses of echoes resulting. Informed analysis of the patterns on the screen allows identification of echoes which are created by the existence of a specific defect such as voids and material discontinuities. More sophisticated methods allow preparation of a pictorial representation of the specimen interior which also identifies defect locations and size.
Generation of an ultrasonic wave is most often done using a piezoelectric crystal to which an impulse voltage is applied. The crystal vibrates mechanically at its resonant frequency in response to the impulse applied, creating an ultrasonic wave. A piezoelectric crystal, if exposed to a mechanical vibration, will also function in reverse to generate a corresponding electrical voltage. Thus, a piezoelectric crystal may be used as a T-transducer; a transmitter of ultrasonic waves, or as an R-transducer; a receiver of ultrasonic waves. A single transducer, designated a TR transducer, may perform both roles by transmitting an ultrasonic pulse for several microseconds followed by several thousand microseconds sensitivity to incoming sound. A pair of transducers may be operated in association such that one "pitch" transducer transmits ultrasonic waves while the other "catch" transducer receives pitch waaves as well as any echoes.
High frequency sound waves propagate poorly in compressible fluids such as air. In the absence of affirmative contact between the transducer and the specimen, an incompressible couplant between the surfaces of these must be used to transmit sound waves therebetween. A film of oil, glycerine, or water or a solid wedge is the usual couplant.
Ultrasonic inspections characteristically require numerous test positions of the transducer with respect to the specimen in order to completely inspect all specimen areas. Movement of the transducer between test sites is a repetitious task, and generates voluminous data. A high degree of automation is desirable, especially if the specimen is located in a hostile environment.
Liquid metal fast breeder reactors have large installed pipes containing hot liquid sodium in radiation areas. Regulations proposed for breeder reactors may require routine weld inspections on specific pipes, with test failure criterion based on defect characteristics such as size.
While defect size is of interest in anticipating specimen failure, the time rate of growth of defects is of greater importance. The historical establishment of failure criterion based on defect characteristics measurable at a single point in time is considered a result of limitations of then-existing inspection art rather than attention to real failure modes. Since an installed pipe moves over time due to thermal expansion and other reasons, it is extremely difficult to inspect a given pore or crack twice, separated over a time interval of perhaps a year, in order to obtain defect growth data. A specific defect cannot be precisely relocated and data obtained once cannot be repeated. Consequently, current practice which bases failure criterion on defect size, can easily require repair of a defect, requiring expensive plant shutdown, which defect may in reality be unstressed and not propagating. Also, a small, but stressed crack which is actually growing, may be considered safe.
If an inspection apparatus capable of precise, repeatable inspection of specific defects could be developed, new inspection failure criterion could be developed which would be both safer and more economic.
Precise data repeatability over a time interval, which reveals defect growth, is of obvious importance to inspection of such specimens as aircraft wing supports. Of further importance to applicability to pipes in nuclear power plants is the degree of automation achievable.
Consequently, it is desired to provide a highly automated ultrasonic inspection apparatus, able to accurately repeat data scans, operable in high temperature and radiation environments and especially adapted to inspection of welds on large installed pipes.